With the growth in the consumer printer market inkjet printing has become a broadly applicable technology for supplying small quantities of liquid to a surface in an image-wise way. Both drop-on-demand and continuous drop devices have been conceived and built. Whilst the primary development of inkjet printing has been for aqueous based systems with some applications of solvent based systems, the underlying technology is being applied much more broadly.
In order to create the stream of droplets, a droplet generator is associated with the print head. The droplet generator stimulates the stream of fluid within and just beyond the print head, by a variety of mechanisms known in the art, at a frequency that forces continuous streams of fluid to be broken up into a series of droplets at a specific break-off point within the vicinity of the nozzle plate. In the simplest case, this stimulation is carried out at a fixed frequency that is calculated to be optimal for the particular fluid, and which matches a characteristic drop spacing of the fluid jet ejected from the nozzle orifice. The distance between successively formed droplets, S, is related to the droplet velocity, Udrop, and the stimulation frequency, f, by the relationship: Udrop=f·S. The droplet velocity is related to the jet velocity, Ujet, via
      U    drop    =            U      jet        -          σ              ρ        ⁢                                  ⁢                  U          jet                ⁢        R            where is the σ the surface tension (N/m), ρ the liquid density (kg/m3) and R the jet's unperturbed radius (m).
U.S. Pat. No. 3,596,275, discloses three types of fixed frequency generation of droplets with a constant velocity and mass for a continuous inkjet recorder. The first technique involves vibrating the nozzle itself. The second technique imposes a pressure variation on the fluid in the nozzle by means of a piezoelectric transducer, placed typically within the cavity feeding the nozzle. A third technique involves exciting a fluid jet electrohydrodynamically (EHD) with an EHD droplet stimulation electrode.
Additionally, continuous inkjet systems employed in high quality printing operations typically require small closely spaced nozzles with highly uniform manufacturing tolerances. Fluid forced under pressure through these nozzles typically causes the ejection of small droplets, on the order of a few pico-liters in size, travelling at speeds from 10 to 50 meters per second. These droplets are generated at a rate ranging from tens to many hundreds of kilohertz. Small, closely spaced nozzles, with highly consistent geometry and placement can be constructed using micro-machining technologies such as those found in the semiconductor industry. Typically, nozzle channel plates produced by these techniques are made from materials such as silicon and other materials commonly employed in micromachining manufacture (MEMS). Multi-layer combinations of materials can be employed with different functional properties including electrical conductivity. Micro-machining technologies may include etching. Therefore through-holes can be etched in the nozzle plate substrate to produce the nozzles. These etching techniques may include wet chemical, inert plasma or chemically reactive plasma etching processes. The micro-machining methods employed to produce the nozzle channel plates may also be used to produce other structures in the print head. These other structures may include ink feed channels and ink reservoirs. Thus, an array of nozzle channels may be formed by etching through the surface of a substrate into a large recess or reservoir which itself is formed by etching from the other side of the substrate.
There are many known examples of inkjet printing. U.S. Pat. No. 5,801,734 discloses a method of continuous inkjet printing. U.S. Pat. No. 3,596,275 discloses methods of stimulating a jet of liquid. US 2006/0092230 discloses a method of charging an insulating ink liquid for use in a continuous inkjet device. U.S. Pat. No. 7,192,120 is representative of a number of patents disclosing novel drop on demand inkjet devices.